Example Using Aspen Plus

Aspen Plus allows you easily to solve this same problem using the Flash2 unit operation. 

Step 1 Start Aspen and choose Template OK. When the window appears, choose General with English Units. In the Run Type (lower right-hand comer), choose Flowsheet. Click OK when the Aspen engine window appears. (This last step is specific to your installation.) 

Step 2 If the bottom of the screen does not show the units, use the View/Model Library 

Step 4 Click on the glasses. This will bring a menu to the left of the screen. The boxes that are red indicate at you still need to supply information. Start at the top and work down, turning the red boxes into blue boxes by filling in the forms. 

Step 6 In the list at the left, choose Property/Specifications and choose the thermodyn- 

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Although ASPEN-Plus is widely used to simulate petrochemical processes, its uses for modeling biomass processes are limited owing to the limited availability of physical properties that best describe biomass components such as cellulose, xylan, and lignin. For example, Lynd et al. (1) used conventional methods to calculate the economic viability of a biom-ass-to-ethanol process. However, with the development by the National Renewable Energy Laboratory (NREL) of an ASPEN-Plus physical property database for biofuels components, modified versions of ASPEN-Plus software can now be used to model biomass processes (2). Wooley et al. (3) used ASPEN-Plus simulation software to calculate equipment and energy costs for an entire biomass-to-ethanol process that made use of dilute-H2S04 acid pretreatment. 

With all these choices, and limited knowledge of your system, you will likely want to use the recommended options and make predictions of vapor-liquid equilibrium using Aspen Plus in order to compare those predictions with experimental data. Chapter 3 presented an example of such a comparison for the ethanol-water system. 

The two basic flowsheet software architectures are sequential modular and equation-based. In sequential modular, we write each unit model so that it calculates output(s), given feed(s), and unit parameters. This is the most commonly used flowsheeting architecture at present, and examples include Aspen+ plus Hysys (AspenTech), ChemCAD, and PROll (SimSci). In equation-based (or open-system) architectures, all equations are written describing material and energy balances as algebraic equations in the form/(x) = 0. This is the preferred architecture for new simulators and optimization, and examples include Speedup (AspenTech) and gPROMS (PSE pic). Each is discussed in turn. 

As in the previous example, the product specifications are met very closely using Aspen Plus . This particular case also shows that profiles match each other very closely. 

Figure 4.B.1 Separation of an acetone, methanol and toluene mixture using three distillation columns example in Aspen Plus V8.4.

In this example, the ammonia reactor loop in Figure 5.3 is simulated using ASPEN PLUS to examine the effect of the purge-to-recycle ratio on the compositions and flow rates of the purge and recycle streams. For the ASPEN PLUS flowsheet below, the following specifications are made ... 

After the simulation file is augmented, the revised simulation is run and the results are sent to Aspen IPE. Note that the ASPEN PLUS and HYSYS.Plant simulators contain menu entries to direct the results to Aspen IPE. For details, the reader is referred to course notes prepared at the University of Pennsylvania (Nathanson and Seider, 2003), which are provided in the file. Aspen IPE Course Notes.pdf, on this CD-ROM. This section presents estimates of equipment sizes and purchase and installation costs using Aspen IPE for two examples involving (1) the depropanizer distillation tower presented on the CD-ROM (either HYSYS —> Separations —> Distillation or ASPEN PLUS Separations Distillation), and (2) the monochlorobenzene (MCB) separation process introduced in Section 4.4, with simulation results using ASPEN PLUS provided on the CD-ROM (ASPEN Principles of Flowsheet Simulation —> Interpretation of Input and Output —> Sample Problem). Just the key specifications and results are presented here. The details of using Aspen IPE for these two examples are presented in the file. Aspen IPE Course Notes.pdf... 

In the propane/isobutane column example, the Aspen Plus files are called Examplel. apwz and Examplel.bkp. The files generated and used in Aspen Dynamics are Examplel. dynfwA Exampleldyn.appdf. Both of these files are needed to run the simulation in Aspen Dynamics. 

Since process simulators are used extensively in commercial practice, I have continued to include process simulation examples and homework problems throughout the text. I now teach the required three-credit, junior-level separations course at Purdue as two lectures and a two-hour conputer lab every week. The computer lab includes a lab test to assess the ability of the students to use the simulator. Although 1 use Aspen Plus as the simulator, any process simulator can be used. Chapters 2. 6, 8,10,12, 13. and 16 include appendices that present instructions for operation of Aspen Plus. The appendices to Chapters 2. 4, 5,15, and 17 have Excel spreadsheets, some of which use Visual Basic programs. I chose to use spreadsheets instead of a higher-level mathematical program because spreadsheets are universally available. The appendix to Chapter 18 includes brief instructions for operation of the commercial Aspen Chromatography simulator—more detailed instruction sheets are available from the author wankat purdue.edu. 

In this example the steady-state simulation of two bioethanol processes is conducted by modifying the model developed by McAloon et al. using Aspen Plus and Microsoft Excel, where the non-random two Hquid thermodynamic model is used [41,43,44]. The plant capacity is considered to be 100 milhon gal per year (378.5 km per year) for both cases. The simulation results shown in Tables 6.2—6.4 provide information on the overall input and output material, component balance, and utihty consumption for both cases. 

The shell and tube heat exchange design using Aspen Plus is done as that in Example 4.4. The process flow sheet and stream table properties are shown in Figure 4.37. The Exchanger details are shown in Figure 4.38. 

Process flow sheet with Stream Table of Example 8.2 using Aspen Plus. 

To study different operating conditions in the pilot plant, a steady-state process simulator was used. Process simulators solve material- and energy-balance, but they do not generally integrate the equations of motion. The commercially-available program, Aspen Plus Tm, was used in this example. Other steady-state process simulators could be used as well. To describe the C02-solvent system, the predictive PSRK model [11,12], which was found suitable to treat this mixture, was applied. To obtain more reliable information, a model with parameters regressed from experimental data is required. 

Several important types of reactions are considered in the following sections. The equations describing each of these systems are developed. The steady-state design of CSTRs with these reactions are discussed, using Matlab programs for hypothetical chemical examples and the commercial software Aspen Plus for a real chemical example. 

The ethylbenzene CSTR considered in Chapter 2 (Section 2.8) is used in this section as an example to illustrate how dynamic controllability can be studied using Aspen Dynamics. In the numerical example the 100-m3 reactor operates at 430 K with two feedstreams 0.2 kmol/s of ethylene and 0.4 kmol/s of benzene. The vessel is jacket-cooled with a jacket heat transfer area of 100.5 m2 and a heat transfer rate of 13.46 x 106 W. As we will see in the discussion below, the steady-state simulator Aspen Plus does not consider heat transfer area or heat transfer coefficients, but simply calculates a required UA given the type of heat removal specified. 

The simulator packages such as Aspen Plus and Hysys may be useful in analyzing distillation column systems to improve recovery and separation capacity, and to decrease the rate of entropy production. For example, for the optimization of feed conditions and reflux, exergy analysis can be helpful. A complete exergy analysis, however, should include both an examination of the exergy losses related to economic and environmental costs and suggestions for modifications to reduce these costs. Otherwise, the analysis is only theoretical and less effective. 

The converged mass and heat balances and the exergy loss profiles produced by the Aspen Plus simulator can help in assessing the thermodynamic performance of distillation columns. The exergy values are estimated from the enthalpy and entropy of the streams generated by the simulator. In the following examples, the assessment studies illustrate the use of exergy in the separation sections of a methanol production plant, a 15-component two-column... 

Instructions on how to use the different software packages (POLYMATH, MATLAB, and ASPEN PLUS) to solve examples. 

The material balance from Problem 3-6 and either ASPEN PLUS or CHEMCAD-III computer software is used to develop the energy balance around each piece of equipment in the ethylene separation section. For example, around distillation column, C-601, the computer program establishes the heat content 6f streams 533, 602, and 603 above a selected datum plane. The distillation calculation indicates the flow rates of the oveiiiead and bottoms streams. The reflux and reboil then indicate the flow rates of the streams that are returned to the column and permits evaluation of the condenser and reboiler duties. In kW " this pan be expressed as... 

In Aspen Plus, solid components are identified as different types. Pure materials with measurable properties such as molecular weight, vapor pressure, and critical temperature and pressure are known as conventional solids and are present in the MIXED substream with other pure components. They can participate in any of the phase or reaction equilibria specified in any unit operation. If the solid phase participates only in reaction equilibrium but not in phase equilibrium (for example, when the solubility in the fluid phase is known to be very low), then it is called a conventional inert solid and is listed in a substream CISOLID. If a solid is not involved in either phase or reaction equilibrium, then it is a nonconventional solid and is assigned to substream NC. Nonconventional solids are defined by attributes rather than molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp, and other multicomponent solid materials. 

Instead, the simulation programs allow the designer to impose constraints on the model. In the preceding example, this would be a constraint that the product flow rate is equal to a target value. Constraints are imposed using controller functions, known as a Design Spec in Aspen Plus or a Set or Adjust in UniSim Design. Controllers are specified either as... 

We are grateful to Aspen Technology Inc. and Honeywell Inc. for permission to include the screen shots that were generated using their software to illustrate the process simulation and costing examples. Laurie Wang of Honeywell also provided valuable review comments. The material safety data sheet in Appendix I is reproduced with permission of Eischer Scientific Inc. Aspen Plus , Aspen Kbase, Aspen ICARUS,... 

As in Example 4, the EXTRACT block in the Aspen Plus process simulation program (version 12.1) is used to model this problem, but any of a number of process simulation programs such as mentioned earlier may be used for this purpose. The first task is to obtain an accurate fit of the liquid-liquid equilibrium (LLE) data with an appropriate model, realizing that liquid-liquid extraction simulations are very sensitive to the quality of the LLE data fit. The NRTL liquid activity-coefficient model [Eq. (15-27)] is utilized for this purpose since it can represent a wide range of LLE systems accurately. The regression of the NRTL binary interaction parameters is performed with the Aspen Plus Data Regression System (DRS) to ensure that the resulting parameters are consistent with the form of the NRTL model equations used within Aspen Plus. 

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